Knowledge of Hematocrit (Hct) is important in several chronic and critical care situations. Presently, Hct measurement requires finger sticks, heel sticks, and blood withdrawals, causing patient trauma as a result of such blood sampling. The development of noninvasive Hct measurement instrument is proposed which will eliminate the trauma of present invasive measurement methods as well as provide immediate results. The method relies upon the sensitivity of the electrical impedance of blood to hematocrit, which differs at high and low frequencies. This principle has been used successfully to measure hematocrit in vitro and in pulse oximetry. The method should be ideal as a screening tool for prospective blood donors as well as for applications in doctors' offices. Since this is a monitoring instrument that provides a continuous measurement, patients in surgery or critical care would benefit from the immediacy of the results compared to the time taken in waiting for lab test results. The tasks of Phase I are directed toward the development and validation of the method. Assuming a successful Phase I, a commercial prototype instrument will be designed and tested in the Phase II.